Method of lowering the carbamate content of cellulose carbamate products using a base/alkali metal salt mixture

ABSTRACT

The carbamate content of cellulose carbamate products is reduced by treating such products with solutions of a base, which solution also contains one or several alkali metal salts, particularly carbonates, sulfates, phosphates, borates and acetates.

BACKGROUND OF THE INVENTION

The invention relates to the lowering of the number of carbamate groupsin products made of cellulose carbamate. As is well known, cellulosecarbamate is a compound formed when cellulose is heated at elevatedtemperature in the presence of urea. Cellulose carbamate is analkali-soluble compound, and this permits it to be converted into theshape of various products by preparing an alkaline solution thereof andthen precipitating the products from the solution in desired form, forexample, as fibers, films, spongy products, etc.

One of the factors which influences the properties of cellulosecarbamate products, and consequently the use thereof, is the carbamatecontent, that is the number of carbamate groups in the cellulose chains.It is known that carbamate groups to a certain extent increases thesensitivity of the products to water and as a consequence has an effecton the wet strength of the products.

It is possible by reducing the number of carbamate groups to influencethe wet strength properties of the products to a desired degree. U.S.Pat. No. 4,583,984 discloses a process for lowering the quantity ofcarbamate by treating the products with alkali solutions. This procedurein principle makes it possible to completely eliminate the carbamategroups, whereby products consisting of regenerated cellulose areobtained.

It has been found, however, that the lowering of the quantity ofcarbamate groups with the aid of alkali has an adverse effect on certainstrength characteristics, particularly in the case of industrial scalewherein maximum fast and efficient removal of carbamate groups is aimedat, for example, using concentrated alkali solutions or high treatmenttemperatures.

SUMMARY OF THE INVENTION

It is accordingly a primary object of the present invention to provide aprocess for the lowering of the carbamate content of products made fromcellulose carbamate without incurring the above mentioned drawbacks.

It is another object of the present invention to provide products madeof cellulose carbamate with the carbamate content thereof reduced inaccordance with the process of the invention.

Other objects and advantages of the present invention will be apparentfrom a reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainlycomprises the lowering of the carbamate content of products made ofcellulose carbomate by treating such products with solutions of a basewhich contain one or several alkali metal salts, particularly alkalimetal carbonates, sulfates, phosphates, borates and acetates.

The base used in the process of the invention should be a stronginorganic or organic base. From the standpoint of price and efficiencyof treatment, the inorganic bases are preferred, namely alkali metalhydroxides, particularly sodium hydroxide. The preferred solvent iswater. The preferred organic base is tetramethylammonium hydroxide. Theconcentration of the base is preferably in the range of 0.5 to 5% byweight.

The alkal metal salts used according to the present invention are thosewhich are highly soluble in basic solutions. Among the salts are thecarbonates, sulfates, phophates, borates and acetates of the alkalimetals. The preferred alkali metal is sodium.

Consequently, the preferred salts according to the present invention areone or more of the salts: sodium carbonate, sodium sulfate, sodiumborate and sodium acetate. The preferred quantity of the salt is 5-35%by weight. This quantity refers to the quantity of dry salt. Thequantities may be correspondingly increased when the salts contain waterof crystallization.

The treatment according to the present invention is preferably performedat elevated temperatures, of from room temperature to 120° C. When lowtemperatures are employed, there is a risk that the treatment time willbecome too long, although lower temperatures can be used. However, inthe case of an industrial process, the treatment time should not be longand the high reaction speed is therefore aimed at by elevating thereaction temperature and/or using a concentrated base solution. The mostsuitable treatment temperature is between about 80°-100° C. and thetreatment time at this temperature may be five seconds to ten minutes.

The treatment is conveniently carried out by immersing the product to betreated, for the desired time, at the desired temperature, in thetreatment solution. The products to be treated may be in anyconceiveable form. From the standpoint of industrial efficiency, it ispreferred that the product be in the form of a continuous product whichis passed through the treatment solution at such speed that the productwill be in contact with the solution for the desired time.

Subsequent to the treatment, the product is washed, for example, withwater. The washing water may contain a small amount of acid in order toeliminate any base that may have remained in the product. A suitableacid for this purpose is acetic acid. The products are then dried innormal manner.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following examples are given to illustrate the present invention.The scope of the invention is not, however, meant to be limited to thespecific details of the examples.

In these examples, the amount of carbamate groups was characterized bymeans of the nitrogen content as measured by means of the so calledKjeldahl method. The method has been described, e.g., in: Snell-Hilton,Encyclopedia of Industrial Chemical Analysis, Interscience Publishers,New York, 1968, Vol. 2, p. 530.

EXAMPLE 1

Cellulose carbamate fibers were prepared by impregnating bleachedsulfite cellulose with urea. Impregnation was accomplished by immersingthe cellulose in liquid ammonia containing 16% urea and 20% waer. Uponimpregnation the cellulose was dried at room temperature to remove theammonia, and thereafter at 100° C. to remove the water. The dry,impregnated cellulose was heated at 140° C. during 3 hrs., whereby itwas converted to cellulose carbamate, which had, after washing withwater, a nitrogen content of 3.4%, calculated on the dry matter.

This cellulose carbamate was activated by soaking it in water for onehour, followed by NaOH addition and dissolving of the cellulosecarbamate products, with agitation at -5° C., during one hour. Thesolution thus obtained was yellowish, and clear. It was filtered anddeaerated by vacuum treatment. The solution thus obtained was spun witha miniature spinning machine through a nozzle piece with 300 holes of 50μm diameter, into a precipitation bath containing 8% sulphuric acid and20% Na₂ SO₄.

The cellulose carbamate sheets thus obtained were treated with anaqueous solution containing 2% by weight sodium hydroxide and 10% byweight sodium carbonate. The treatment was carried out by immersing thefibers at 90° C. in the treatment solution, in which they were kept fortreatment periods of different lengths. After treatment, the fibers werewashed with water containing some acetic acid.

The characteristics of the fibers used for starting material and ofthose which were obtained are set forth in Table 1.

                  TABLE I                                                         ______________________________________                                                          Treatmemt time, s                                           Characteristic      0      30     60    120                                   ______________________________________                                        Nitrogen content, % 2.3    1.20   0.81  0.41                                  dtex                1.1    2.12   2.08  2.02                                  Tensile strength when dry, cN/dtex                                                                2.43   2.33   2.46  2.35                                  Breaking elongation, %                                                                            8.6    14.9   14.7  14.2                                  Loop strength, cN/dtex                                                                            0.31   0.45   0.42  0.44                                  ______________________________________                                    

EXAMPLE 2

As in Example 1, cellulose carbamate fibers were produced, and treatedwith an aqueous solution containing 2% by weight sodium hydroxide and20% by weight sodium carbonate. The treatment was carried out at 90° C.,using various treatment times. After treatment, the fibers were washedwith water containing acetic acid, and dried.

The characteristics of the fibers thus obtained are set forth in Table2.

                  TABLE 2                                                         ______________________________________                                                          Treatment time, s                                           Characteristic      0      30     60    120                                   ______________________________________                                        Nitrogen content, % 2.08   0.91   0.47  0.32                                  dtex                1.53   1.59   1.58  1.54                                  Tensile strength when dry, cN/dtex                                                                2.30   2.52   2.39  2.44                                  Breaking elongation, %                                                                            9.2    11.4   11.9  11.4                                  Loop strength, cN/dtex                                                                            0.36   0.39   0.39  0.40                                  ______________________________________                                    

EXAMPLE 3

As in Example 1, cellulose carbamate fibers were produced, and treatedwith an aqueous solution containing 4% by weight sodium hydroxide adn20% by weight sodium carbonate. The treatment was carried out at 100°C., using various treatment times. After treatment, the fibers werewashed with water containing acetic acid.

The characteristics of the fibers thus obtained are set forth in Table3.

                  TABLE 4                                                         ______________________________________                                                          Treatment time, s                                           Characteristic      0      30     60    120                                   ______________________________________                                        Nitrogen content, % 3.03   0.56   0.41  0.26                                  dtex                1.60   1.79   1.81  1.76                                  Tensile strength when dry, cN/dtex                                                                2.59   2.40   2.49  2.43                                  Breaking elongation, %                                                                            8.6    16.5   15.8  16.4                                  Loop strength, cN/dtex                                                                            0.29   0.57   0.655 0.64                                  ______________________________________                                    

EXAMPLE 5

As in Example 1, cellulose carbamate fibrs were produced, and treatedwith an aqueous solution containing 4% by weight sodium hydroxide and26% by weight sodium sulfate. The treatment was carried out at 100° C.,using various treatment times. After treatment, the fibers were washedwith water containing acetic acid, and dried.

The characteristics of the fiber thus obtained are set forth in Table 5.

                  TABLE 5                                                         ______________________________________                                                            Treatment time, s                                         Characteristic        0      30       60                                      ______________________________________                                        Nitrogen content, %   3.03   0.58     0.40                                    dtex                  1.60   1.77     1.70                                    Tensile strength when dry, cN/dtex                                                                  2.59   2.33     2.47                                    Breaking elongation, %                                                                              8.3    16.1     14.6                                    Loop strength, cN/dtex                                                                              0.29   0.63     0.49                                    ______________________________________                                    

COMPARISON EXAMPLE 1

As in Example 1, cellulose carbamate fibers were produced, and treatedwith a 2% by weight aqueous solution of sodium hydroxide. The treatmentwas carried out at 100° C., using various treatment time. Aftertreatment, the fibers were washed as in Example 1.

The characteristics of the fibers thus obtained are set forth in Table6.

                  TABLE 6                                                         ______________________________________                                                          Treatment time, s                                           Characteristic      0      30     60    120                                   ______________________________________                                        Nitrogen content, % 2.31   0.89   0.59  0.34                                  dtex                2.15   2.33   2.41  2.26                                  Tensile strength when dry, cN/dtex                                                                2.28   1.74   1.72  1.95                                  Breaking elongation, %                                                                            10.4   23.9   25.3  20.6                                  ______________________________________                                    

COMPARISON EXAMPLE 2

Comparison Example 1 was repeated, using the treatment temperature 90°C. The results are presented in Table 7.

                  TABLE 7                                                         ______________________________________                                                          Treatment time, s                                           Characteristic      0      30     60    120                                   ______________________________________                                        Nitrogen content, % 2.22   1.18   0.79  0.45                                  dtex                2.07   2.39   2.39  2.35                                  Tensile strength when dry, cN/dtex                                                                2.36   1.74   1.75  1.88                                  Breaking elongation, %                                                                            8.6    24.2   24.8  27.4                                  ______________________________________                                    

Comparison Examples 1 and 2 reveal that when cellulose carbamate fibersare treated with a basic solution which does not contain soluble alkalimetal salts as in Examples 1-5, the strength of the product is impairedand the elongation considerably increased.

While the invention has been set forth with respect to specifictreatment solutions, it is apparent that variations and modificationscan be made without departing from the spirit or scope of the invention.

What is claimed is:
 1. Method of lowering the carbamate content ofcellulose carbamate without adversely affecting strength and elongationcharacteristics thereof, which comprises contacting cellulose carbamatewith a solution of an alkali or organic base and also containing insolution at least one soluble alkali metal salt.
 2. Method according toclaim 1 wherein said alkali metal salt is an alkali metal carbonate,sulfate, phosphate, borate or acetate.
 3. Method according to claim 1wherein said base solution is in aqueous 0.5-5% sodium hydroxidesolution.
 4. Method according to claim 2 wherein said base solution is0.5-5% aqueous sodium hydroxide solution.
 5. Method according to claim 1wherein the content of alkali metal salt in the solution is about 5-35%by weight, calculated as dry salt.
 6. Method according to claim 4wherein the alkali metal salt content of said solution is about 5-35% byweight, calculated as dry salt.
 7. Method according to claim 1 whereinsaid alkali metal salt is a sodium salt.
 8. Method according to claim 4wherein said alkali metal salt is a sodium salt.
 9. Method according toclaim 1 wherein the thus treated cellulose carbamate is then washed. 10.Method according to claim 9 wherein the washing is with water. 11.Method according to claim 9 wherein the washing is effected with watercontaining acetic acid.
 12. Method according to claim 1 wherein saidbase is in solution in an amount of 2-5% by weight.
 13. Method accordingto claim 2 wherein said base solution is a 2-5% aqueous sodium hydroxidesolution.